Field of the Invention
The invention lies in the field of electronic reproduction technology and relates to a method for the reproduction of spot colors with a combination of primary printing inks and secondary printing inks. Sport colors, also referred to as special colors or decorative colors, are used in reproduction and printing technology in order to reproduce product-specific colors or company-specific colors in printed products. Examples of spot colors are the unique blue on Nivea® products or the special blue used in the design mark HEIDELBERG® in the company logo of Heidelberger Druckmaschinen AG of Germany. Spot colors have to be reproduced with very good color fidelity, since they are often a significant identifier of the products and the corporate identity of companies and therefore have a high recognition and association value.
In reproduction technology, printing originals for printed pages that contain all the elements to be printed such as texts, graphics, and images are produced. For color printing, a separate printing original is produced for each printing ink and contains all the elements which are printed in the respective color. For four-color printing, these are the printing inks cyan (C), magenta (M), yellow (Y) and black (K). The printing originals separated in accordance with printing inks are also referred to as color separations. The printing originals are generally scanned and, by using an exposer, are exposed onto films, with which printing plates for printing large editions are then produced. Alternatively, the printing originals can also be exposed directly onto printing plates in special exposure devices, or they are transferred directly as digital data to a digital press. There, the printing-original data is then exposed onto printing plates, for example with an exposing unit integrated into the press, before the printing of the edition begins immediately thereafter. There are also digital presses which do not need any printing plates since, for example, they operate in accordance with the electro-photographic printing principle or with inkjet printing.
According to the current state of the art, the printing originals are reproduced electronically. In this case, the images are scanned in a color scanner and stored in the form of digital data. Texts are generated with text processing programs and graphics with drawing programs. Using a layout program, the image, text and graphic elements are assembled to form a printed page. The data from a plurality of printed pages is combined with data from further elements, such as register crosses, cutting marks and folding marks and print control fields, to form printing originals for a printed sheet. The data formats largely used nowadays to describe the printing originals are the page description languages PostScript and PDF (portable document format). In a first step, the PostScript or PDF data is converted in a raster image processor (RIP) into color separation values for the color separations C, M, Y and K before the recording of the printing originals. In the process, for each image point, four color separation values are produced as tonal values in the value range from 0 to 100%. The color separation values are a measure of the color densities with which the four printing inks cyan, magenta, yellow and black have to be printed on the printing material. In special cases, in which printing is carried out with more than four colors, each image point is described by as many color separation values as there are printing inks. The color separation values can be stored, for example, as a data value with 8 bits for each image point and printing ink, with which the value range from 0% to 100% is subdivided into 256 tonal value steps.
The colors of the elements of a printed page are usually not defined directly in the CMYK color system of the printing inks but, as a rule, in another color system. For example, the images are broken down in a scanner by means of color filters into the color components red, green and blue (RGB), that is to say into the components of a three-dimensional color space. Therefore, before the printing originals are recorded on color separation films or printing plates, the image data has to be transformed from the RGB color space of the scanner into the CMYK color space of the printing process to be used.
Such color space transformations are needed in reproduction technology, since the devices and processes used have specific restrictions and special features in the display and reproduction of the colors and these properties are different in all the devices and processes. Therefore, for different devices and processes such as scanners, monitors, proofers, and printing processes and so on, there are different color spaces, which in each case describe the color characteristics of the device or process optimally and which are designated device-dependent color spaces. In addition to the device-dependent color spaces there are also device-independent color spaces, which are based on the human viewing characteristics of what is known as a standard observer. Such color spaces are, for example, the XYZ color space defined by the CIE standardization commission (Commission International d'Éclairage) or the Lab color space derived from this, the Lab color space having gained wider acceptance in the technology. If it is wished to know whether two colors will be perceived as identical or different by the human eye, then the measurement of the XYZ or Lab color components is sufficient for this purpose. The Lab color components form a three-dimensional color space having a lightness axis (L) and two color axes (a, b), which can be imagined in the plane of a color circle through whose center the lightness axis runs. The Lab color components are related to the XYZ color components via nonlinear conversion equations.
A device or color processing process can be characterized with respect to its color characteristics by all the possible value combinations of the associated device dependent color space being assigned the Lab color components which a human sees in the colors produced with these value combinations. For a printing process, the various CMYK value combinations in each case produce a different printed color. Using a color measuring instrument, the Lab components of the printed colors can be determined and assigned to the CMYK value combinations. Such an assignment, which relates the device dependent colors produced by a device or process to a device independent color space (XYZ or Lab) is also designated a color profile, an output color profile in the case of a printing process. The definition and data formats for color profiles have been standardized by the ICC (International Color Consortium—Specification ICC. 1:2003-09). In an ICC color profile, the association between the color spaces is stored in both directions, for example the assignment Lab=f1 (CMYK) and the inverse assignment CMYK=f2 (Lab). The assignments defined by a color profile can be implemented with the aid of a look-up table. If, for example, the CMYK color components of a printing process are to be assigned the Lab color components of a printing process, the look-up table must have sufficient space, in which the associated Lab color components are stored for each possible value combination of the CMYK color components. However, this simple association method has the disadvantage that the look-up table can become very large. If each of the CMYK color components has 256 total value steps, there are 2564=4,294,967,296 possible value combinations of the CMYK color components. The look-up table must therefore have 4,294,967,296 storage cells with six bytes word length (two bytes each for L, a, b). In order to reduce the size of the look-up table, a combination of look-up table and interpolation method is therefore used in order to describe a color profile and to implement an appropriate color space transformation. In the look-up table, only the associated Lab components for a coarse, regular grid of reference cells in the CMYK color space are stored. For CMYK value combinations which lie between the grid points, the Lab color values to be assigned are interpolated from the stored Lab color values of the adjacent reference points.
In order to transform the RGB color values of a scanned image into the CMYK color values of the printing process, the association tables of a scanner color profile and of a printing process color profile are applied one after the other. First of all, the RGB color values are converted by means of the scanner color profile into the device independent Lab color values. These are then converted by means of the printing process color profile into the CMYK color values of the printing process. Alternatively, the two color profiles can also be linked in advance to form a conversion color profile, whose tables contain the direct assignment of the RGB color values to the CMYK color values. Since the RGB color values and the CMYK color values which result in the same Lab color values are in each case associated with one another via the device independent Lab intermediate color space, the associated colors are perceived as visually identical. However, this assignment is only possible within the restricted color gamut which can be printed with the CMYK printing inks. If the color gamut of the RGB color values of a scanned image is larger, such as is the case for color slides, for example, it is not possible for all the colors of the image to be reproduced exactly with the CMYK printing inks. Then, color gamut mapping is incorporated in the assignment of the Lab color values to the CMYK color values of the printing process, compressing the colors to the printable color gamut. In this case, in particular the unprintable light and saturated colors are converted into similar colors in the marginal area of the color gamut of the printable colors, so that the result is an overall harmonious color impression without subjectively perceived color distortions. These restrictions can be reduced if, instead of the standard printing inks for CMY and K, special CMYK printing inks which have a higher chroma or blackness are used. Another solution is printing with further colored printing inks in addition to the CMYK printing inks, for example with three further printing inks R, G, and B, which widen the printable color gamut in the region of the red, green and blue hues.
The associations specified in the color profiles between device dependent color spaces and a device independent color space (e.g. Lab) are also used for the reproduction of spot colors, according to the prior art. In the PostScript or PDF page description of a printed page, the spot color is designated by a name and/or a number and, in addition, the Lab values of the spot color are specified. In order to simplify the application, there are standardized spot color systems, for example systems from Pantone Inc., which provide a large selection of spot colors with defined designations and the associated Lab values. The Lab values of the spot colors specified in the page description are then converted by the raster image processor (RIP) or another suitable application into the CMYK color values via the printing process color profile. Alternatively, for the spot colors, in addition to the designation, the proportions of the CMYK printing inks can also be specified directly in the PDF page description. The spot colors simulated in this way are, however, subject to an increased extent to the process fluctuations of the printing process and change accordingly. Since, in the case of the spot colors, faithful color reproduction in particular is involved, this simple solution is often not sufficient either because of the restricted color gamut of the printable colors. Therefore, spot colors are also reproduced as separate color separations, which are printed with specifically mixed additional printing inks. Such a solution is complicated and, in addition, is restricted to two to three spot colors in a printed product.
If, in the printing process, because of the restriction of the color gamut of the CMYK printing inks, further colored printing inks are already used, then the additional printing inks can also be used to simulate spot colors as well. In the following text, to make a clear distinction, the CMYK printing inks will be designated “primary printing inks” and the further color printing inks as “secondary printing inks”. In such a system which, in addition to the primary printing inks C, M, Y and K, also uses the secondary printing inks red, green, blue (R, G and B), their pigments being considerably more colored than the corresponding mixed colors of the primary printing inks, it is possible to achieve better reproduction of the spot colors and, at the same time, improved process stability in the print. By using, for example, only three secondary printing inks, many different spot colors can be reproduced, and it is not necessary for mixed printing inks corresponding to all the spot colors to be kept in stock in a print shop. In addition, a shorter setup time of the press is achieved, since extra cleaning cycles of the printing units when changing from one spot color to another are avoided. FIG. 1 shows, in an xy color locus diagram, the color gamut 1 of the primary printing inks and the expanded color gamut 2 of the seven-color printing system comprising the primary printing inks and the secondary printing inks, for comparison.
European patent EP 0 131 145 B1 describes a printing process which operates with the seven printing inks yellow, orange-red, magenta red, violet-blue, cyan blue, green and black. If it is intended to print on printing materials which are not white, printing is additionally carried out with the eighth printing color white. An image to be reproduced is subdivided into small subareas and, in each subarea, proportional areas of a maximum of four printing inks are printed, in each case two of the colored printing inks and black and also, if appropriate, white. A special feature of the process is that the proportional areas are printed beside one another and not over one another. The area proportions to be printed are derived from three scanner color signals by means of simple operations such as subtraction, comparison, minimum determination. For instance, the proportion of white is determined as the smallest value of the scanner signals. After this value has been subtracted from all three scanner signals, the remaining two scanner signals determine which colored printing inks are printed with which area proportions.
U.S. Pat. No. 5,734,800 describes a six-color printing process in which printing is carried out with highly pigmented printing inks yellow, orange, magenta, cyan, green and black (YOMCGK). For the printing inks yellow, orange, magenta, use is preferably made of fluorescent colors, in order to increase the brilliance and saturation of the colors and to expand the printable color gamut. In order to determine what proportions of the printing inks are required in order to reproduce a specific color defined by the CIE color components XYZ, color panels are printed whose color areas are in each case superimposed tonal value steps of two printing inks adjacent in the color circle, that is to say color panels are printed for the combinations YOK, OMK, MCK, CGK and GYK. The printed color areas are measured calorimetrically and the XYZ values determined are plotted in a three-dimensional CIE diagram. For an image to be reproduced, the XYZ values of the image colors are likewise determined with a color scanner and compared with the XYZ values of the color panels. The assignment of the proportions of printing inks to the image colors is carried out on the basis of the XYZ values which best agree. International PCT publication WO 02/071739 A1 (corresponding to patent application publication US 2004/114163 A1) proposes a printing ink system in which printing is carried out with six printing inks. In addition to the four primary printing inks CMYK, the two secondary printing inks orange and blue or orange and green are provided. Scanned images are printed only with the primary printing inks, and elements which have a spot color are printed with the secondary printing inks orange and one of the two secondary printing inks blue and green and also, in supplementary fashion, with one or two of the primary printing inks. No statements are made as to the method by which the proportions of the printing inks with which a specific spot color is to be reproduced are determined.
Generally applicable methods for determining the proportions of primary printing inks and arbitrary secondary printing inks for the reproduction of spot colors with high color fidelity with, at the same time, high stability of the printing process are not known at present.